Reversible single crystal photochemistry and spin state switching in a metal-cyanide complex

Manipulating the physical properties of solid matter using only photons is a major challenge in materials science. In this study, we present the photochemistry occurring in a single crystal of a simple cyanide complex, K ₄ [Mo ^III (CN) ₇ ]·2H ₂ O. Upon exposure to visible light at different wavelengths, a reversible breaking and reformation of dative bonds is triggered, resulting in a photoswitching of the Mo ^III coordination geometry between 6- and 7-coordinate. This transformation, in turn, induces a spin state change. The observed solid-state photochemical reactivity is robust, quantitative and occurs at a record-high temperature. It paves the way for the development of new photo-switchable high-temperature magnets and nanomagnets.